(1) Field of the Invention
The present invention relates to helical antennas. More particularly the invention pertains to a broadband, phased array, quadrifilar helical antenna that must form particular patterns and coverage at particular frequency ranges on the horizon, near the horizon and overhead.
(2) Description of the Prior Art
In general, previous one short element quadrifilar helix arrays could not provide simultaneous broadband (225 MHz to 400 MHz) impedance, and broadband and broadbeamed cardioid shaped pattern performance. A quadrifilar helix antenna presently used by the Navy is mounted on a mast for satellite coverage. It has a good broadband match since its elements are longer than one wavelength. Broadband matched overhead patterns are obtained for this antenna by making the antenna a 15/8 turn quadrifilar helix. However, the antenna is long and is designed to operate up to 320 MHz. Below one wavelength, a shorter 3/4 turn quadrifilar helix also has broad beamed patterns. However, near one wavelength, the patterns start to split overhead. To provide acceptable overhead patterns from 225 MHz to 400 MHz, this antenna would have to operate below one wavelength with an expected broadband match, after impedance matching, of about 7:1 VSWR. VSWR is Voltage Standing Wave Ratio which describes the amount of power transmitted (or received) by the antenna and it is 1:1 for an ideal case.